Electron multiplier



Feb. 11, 1941. RAJCHMAN EI-AL 2,231,682

ELECTRON MULTIPLIER Filed Oct. 30, 1957 3maentors Patented Feb. 11, 194-1 UNITED STATES PATENT OFF-ICE ELECTRON MULTIPLIER Jan A. Rajchman, Philadelphia, and Eugene W.

Pike, Swarthmore, Pa., assignorstoRadiojCorporation of America, a corporation of Delaware, Application October so, 1937, .Serial No. 171,916

6 Claims. (Cl. 250-175) This invention relates to electron multipliers (i. e., electron discharge tubes of the type wherein amplification of a primary electron stream, such, for example, as is emitted by a thermionic cathode or by a photosensitive surface exposed to light, is accomplished through utilization of the phenomenon of secondary emission), and has special reference to the provision of improvementsin electron multipliers of the type operable without the use of auxiliary electron-lens systems. Discharge tubes of the class to which the invention particularly relates are well exemplified in copending applications Serial No. 107,955 and Serial No. 144,825 of Edward G, Ramberg.

Early attempts to provide an electron multiplier operable without either an auxiliary magnetic or electrostatic lens system did not meet with a great degree of success, principally because, in trying to accelerate the electrons from a source toward the next succeeding multiplying electrode, the electrons would miss that electrode and impinge on a multiplying electrode beyond that on which it was desired that they should impinge. As a consequence, the amplification was lowered. The provision of auxiliary insulating plates for physically guiding the electrons along predetermined paths in such tubes complicates their construction without a proportionate increase in overall efiiciency.

Ramberg, by providing a tube having electrodes of a novel design and arrangement, and maintained at definite potentials, obviates the above described and certain other disadvantages inherent in electrostatically controlled electronmultipliers of the prior art. One trouble, however, encountered in the operation of the Ramberg electron multiplier is that when the electron stream has been increased by several stages of electron multiplication, positive ions may be generated. This phenomenon has been observed to exist even in the most thoroughly evacuated multi-stage tubes. These positively charged particles flow in a direction opposite to the general direction of the electron stream and may im-. pinge upon the primary-electron emitting cathode, or upon an intermediate secondary-electron emitter where they release still more electrons. The electron current may thus be augmented to such a degree as to render it uncontrollable. This building up of the electron current is limited only by the so-called space charge effect and may result in an equilibrium of the output current, i. e., the output current may no longer accurately reflect variations in the input current.

Accordingly, a principal object of the present invention is to provide an improved electron multiplier wherein the output current is directly proportionalto the input current and this, too, substantially irrespective of the intensity of the electron stream impinging upon the outputelectrode.

Another and important object of the present invention is to provide an electron multiplier wherein ions generated in the residual gas by the passage of the electron beam are entrapped and dissipated by barriers near their area of origin. 10

Another object of the invention is to provide an electron multiplier wherein, by reason of a novel design and relative arrangement of the electrodes, the electric fields adjacent the multiplying electrodes will exhibit a greater intensity, 15 causing saturation and hence optimum effective emission of secondary electrons.

Another object of the invention is to provide an electron multiplier wherein the space charge effect and spreading of the electron beam is less :20 pronounced than in analogous prior art devices.

Still another object of this invention is to provide an electron multiplier wherein dark current (current which flows when the tube is energized but the cathode not illuminated) is minimized. 7

Other objects and advantages, together with certain details of construction, will be apparent and the invention itself will be best understood by reference to the following specification and 30 to the accompanying drawing, wherein:

Figure 1 is a partly diagrammatic longitudinal sectional view of an electron multiplier constructed in accordance with the principle of the invention and exemplifying the manner in which 35 the several electrodes are energized when the device is utilized for certain of the purposes to which it is adapted, and

Figure 2 is a longitudinal sectional view of an electron multiplier showing an electron multio plier having electrodes of an alternative construction.

Like reference characters designate the same or corresponding parts in both figures.

Fig. 1 shows a highly evacuated envelope T, 5 which is preferably, though not necessarily, in the form of a cylindrical glass tube having a long central axis indicated in the drawing by dotted line xa:. (Line xa: may comprise an axis of symmetry of the tube or envelope T; the herein 50 described electrode structure, however, has neither an axis nor a plane of symmetry.) Tube T contains a pair of terminal electrodes of a con tour later described, suitably mounted at opposite ends of this axis and comprising a photosensitive cathode I and an anode or collector electrode II. Mounted in spaced relation on opposite sides of this axis, intermediate the cathode I and anode II, is an upper set of L-shaped multiplying electrode plates 2, 4, 6, 8 and I0, respec tively, and a lower duplicate set of multiplying electrode plates 3, 5, I and 9, respectively. Electrodes 2 to H1, inclusive, may be formed, for example, of silver suitably coated with a substance which is the equivalent of caesium, to render them capable of emitting a copious flow of secondary electrons.

Preferably, the distance measured between a point on one multiplying electrode to a corresponding point on the next adjacent electrode in the same set should be less than the distance be- Excellent performance has been achieved in a tube wherein The long legs m (they are so designated in both figures) of the L-shaped multiplying electrodes are inclined toward the cathode, and the free or terminal edges of these long legs preferably extend to (or, in the case of Fig. 2, beyond) equally spaced points m along the central axis :ra:. It will be noted that the axis a:-a: comprises the -median line of the electron path which extends between the cathode and anode.

The short legs n of these L-shaped electrodes extend in the general direction of the anode II and terminate at points 11. which, it will be noted, are removed substantially equal distances 'from the said axis :c-zr.

60 The electrodes I to II, inclusive, are arranged in staggered relation, and it will be observed that the long leg m of each L-shaped multiplying electrode, say electrode 8, Fig. 1, intersects a line e drawn between the terminals of the short legs n of the next two preceding multiplying electrodes (electrodes 6 and 1). Considered from another aspect, the long leg of each L extends beyond the short leg n of the next preceding L of the same (say, the upper) set.

The terminal electrodes, 1. e., the primary electron emitting cathode I and the collecting electrode or anode II, are of a contour such as to ensure a desired distribution of the electrostatic field adjacent these electrodes when the device is energized. The illustrated modified L- shape contour of electrodes I and II corresponds to the contour of the equipotential surface which would be present midway, electrically, between two adjacent multiplying electrodes (in the same upper or lower set) in a'device having an infinite number of multiplying electrodes. The particular forms of theelectrode plates I and I I of the illustrated embodiments of the invention were determined by constructing 55 a scale model in' metal of the electrodes 2 to I0,

inclusive, and experimentally plotting the equipotential line of the potential applied to one of the central electrodes (say electrode 5) when the entire model was energized at the relative potentials to be applied tothe corresponding electrodes of the completed electron multiplier.

Section lm of the cathode is of foraminous construction to permit light from an external source, exemplified in Fig. 1 by the lamp Y and lens Z, to impinge upon the photosensitive surface of the cathode section In.

The long section IIm of the anode is preferably ofimperforate construction; it is upon this portion of the anode that the secondary electrons are collected.

As in the case of the Ramberg electron multiplier, the potential distribution required to ensure optimum performance may be expressed by the mathematical series IV, 2V, 3V, 4V, 5V, 6V, etc., where IV is the potential drop between the primaryelectron source and the first target or multiplying electrode, and 2V, 3V, 4V, etc., represent the potential drop between the respective succeeding electrodes, in point of electron travel, and said source.

Referring to Fig. 1: For the purpose of providing such a potential distribution, the cathode I may be connected to the negative terminal V of a. direct current source, exemplified in the drawing by a resistor R, and the first multiplying electrode, i. e., electrode 2, connected to a point IV somewhat more positive. The other electrodes 3 to II, inclusive, in the order of their numbers, are shown connected to successively more positive points 2V to IOV on the resistor.

The reference characters IV, 2V, 3V, 4V, etc., given to the several points on resistor R, will be understood to indicate that the voltage drop between a given electrode and the cathode is the designated whole number multiple of the drop existing between the cathode I and the first multiplying electrode 2. Thus in the tubes of both Figs. 1 and 2, where the potential drop between the first multiplying electrode 2 and cathode I is volts, the drop between electrodes 3 and I should preferably be 200 volts, that between electrodes 4 and I, 300 volts.

If a beam of light, say ofvarying intensity, is caused to impinge upon the first lower electrode I, photo-electrons will be emitted in a quantity determined by the instantaneous intensity of the light beam. 'I'hese photo-electrons will be accelerated toward the first upper" electrode 2 and, because of the described design, relaive arrangement, and voltage distribution, will impinge upon this first multiplying electrode. The photo-electrons striking electrode 2 will cause the emission of secondary electrons, the number of secondary electrons emit-ted being dependent, in part at least, upon the magnitude of the potential between it and the'cathode.

The next electrode in point of electron travel is the second lower electrode 3. The trajectory of secondary electrons from the first multiplying electrode 2 is such that they impinge upon the cupped surface of the second multiplying electrode 3. Here again, a multiplication, by reason of secondary emission, is secured and this is repeated in any numberof stages until the amplified stream of secondary electrons is collected upon the tilted long leg lIm of the output electrode I I and caused to flow in a utilization circuit exemplified in the drawing by the resistor 1" included between the output electrode II and the positive terminal IDV of the potential divider.

It has previously been pointed out that the long leg m of each of the L-shaped multiplying electrodes extends beyond the short leg 1!. of the next preceding L-shaped electrode of the same set. Thus, that terminal edge of section m of electrode 4 which lies transverse of the line :ca: extends beyond the parallel terminal edge of leg n on electrode 2. This construction augments the electrostatic field in the cup of the next preceding electrode of the same set and thereby provides a higher eiiective emission or saturation of the secondary electron emission from that electrode. Another and very important result of this construction is that ions generated in the residual gas by the passage of the electron beam will be promptly arrested by theibarriers which this construction affords. Thus, ions generated in the space between the last multiplying electrode Ill and the anode II will be collected by the long legs m of electrodes 9 and ID. This prevents internal feed-back and provides a tube having a more nearly linear response, greater stability, and smaller dark current (i. e., current which flows when the multiplier is energized but the cathode not illuminated).

It is not essential that the electrodes be of the cupped or curved L-shape construction shown in Fig. 1. Referring to Fig. 2, they may, for example, comprise long and short plane surfaces m and n, respectively, joined at substantially right angles to each other.

In some cases it is desirable to extend the long legs m of the multiplying electrodes a somewhat greater distance beyond the terminals of the short legs n of the preceding electrodes than is indicated in Fig. 1. Such modification is shown in Fig. 2, wherein each of the long legs m extends beyond the longitudinal axis a:a: of the envelope T.

Other modifications of the invention, such, for example, as the substitution of a thermionic cathode for a photosensitive one, and the addition of a control grid or one or more auxiliary grids, will suggest themselves to those skilled in the art. It is to be understood, therefore, that the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the prior art and by the spirit of the appended claims.

What is claimed is:

1. An electric discharge device comprising an evacuated envelope containing a cathode, an anode and a plurality of sets of substantially L- shaped multiplying electrodes mounted on opposite sides of the median line which spans the space between said cathode and anode, the long legs of said Ls extending from said median line with their free ends inclined toward the cathode and the short legs of said US extending in the direction of the anode and terminating at points spaced from said median line.

2. The invention as set forth in claim 1 and wherein the long leg of each L extends beyond the terminal of the short leg of the next preceding L of the same set.

3. An electric discharge device comprising an evacuated envelope having a long axis, a plural-. ity of sets of multiplying electrodes mounted in staggered relation on opposite sides of said axis, said electrode sets comprising an assembly having neither an axis nor a plane of symmetry, a cathode and an anode substantially enclosing opposite endsof said electrode assembly, said cathode and anode each having a contour which corresponds to the contour of the equipotential surface which would be present midway, electrically, between two adjacent multiplying electrodes of the same set in a device having an infinite number of multiplying electrodes.

4. The invention as set forth in claim 3 wherein said multiplying electrodes have concave surfaces which extend along the said envelope axis and wherein said cathode and anode are substantially L-shaped.

5. An electron multiplier comprising a primary cathode, a collector electrode, and a plurality of secondary cathodes between said primary cathode and said collector electrode and mounted in staggered relation in two rows, said secondary cathodes having opposed curved concave emissive surfaces the generatrices of which are parallel to a medial plane passing between said rows, and the end of each secondary cathode toward said collector electrode being spaced from the longitudinal axis of the opposite row of secondary cathodes a distance greater than the spacing between the end thereof toward said primary cathode and said axis of the opposite row of secondary cathodes.

6. An electron multiplier comprising a primary cathode, a collector electrode and a plurality of secondary cathodes mounted in staggered relation on opposite sides of the median line which spans the space between said primary cathode and said collector electrode, each of said secondary cathodes having an intermediate emissive portion and opposite emissive end-portions, one of the emissive end-portions being inclined toward said cathode and extending to said median line, and the other of said emissive end-portions being inclined toward said collector electrode and terminating short of said median line.

JAN A..RAJCHNLAN. EUGENE W. PIKE. 

